SCADA systems (SCADA: supervision, control and data acquisition) are generally known. They are based on a conventional client-server application and for this purpose have so-called OPC clients and OPC servers. The OPC clients and OPC servers facilitate a standardized and open data communication, in order for example to obtain access to closed process data, also referred to as process variables, of subordinate data sources. OPC is an acronym for “OLE for process control”. The objective of an OPC server is amongst other things to assist an OPC client in the reading of process variables, changing of process variables and monitoring of process variables. As a rule the OPC server carries out these objectives following a request by the OPC client. One such SCADA system is described for example in the textbook “Automatisieren mit SPS-Theorie and Praxis, O. Wellenreuther et al., Vieweg+Teubner, 4th edition, 2009”.
In order to be able to ensure that process control systems operate reliably and in a fail-safe manner in particular in the monitoring of critical states, for example in the monitoring of road tunnels, specific system components are designed to be redundant.
US 20030012693 A1 discloses a redundant process control system which has an OPC client, a plurality of OPC servers and a plurality of data sources connected to the OPC servers. In order to facilitate a redundant operation, the OPC client has a redundancy manager which receives and evaluates process data from a plurality of different data sources. The data sources which transfer data to the OPC client are connected by means of one or more OPC servers to the OPC client. In a further known redundant process control system at least one data source is connected by means of two redundantly operated OPC servers to the OPC client. The selection of a suitable OPC server is made by the redundancy manager of an OPC client.
A similar system is known from U.S. Pat. No. 7,451,215 B2. The known process control system has two redundantly operated OPC servers which function as the main or auxiliary OPC server. Both OPC servers are connected to a data source which makes process variables available for an OPC client. The decision as to which OPC server functions as the main OPC server is made by the OPC client.